(1) Field of the Invention
The present invention relates to an organic electroluminescent device, and more particularly, to an organic electroluminescent device having a color conversion layer.
(2) Description of the Prior Art
In present, the full-color technology used in an organic is electroluminescent display is classified into three types: (1) red, green, and blue light sources placed side-by-side pattern, (2) white light source through each of red, green, and blue color filters, and (3) color change media (CCM) replacing the color filters to convert blue light into other color lights. A brief introduction follows.
FIG. 1A shows an organic electroluminescent display 10a fabricated by placing red, green, and blue sub-pixels side-by-side pattern. As shown, blue, green, and red emissive layers 11, 12, 13 are evaporated on the substrate 14, i.e. heating the red, green, or blue emissive material in vacuum, and respectively depositing them in a predetermined position on the substrate 14 via photomask. An advantage is that these emissive materials are able to appear their color fully and acquire maximum performance. But a disadvantage is, after a long term use, a color shift occurs due to the difference in life between the emissive layers 11, 12, 13. In addition, reduction in pixel size may result in harder mask-making so as to reduce the process yield.
Referring to FIG. 1B, based on the way of white light source through color filters, an organic electroluminescent display 10b uses a white organic electroluminescent device (OELD) 15 as the light source and has a color filter 16 adhered on the light-exit surface of the white OELD 15. The red, green, or blue lights are generated when the white light goes through the color filter 16. Thus, each organic layer of the white OELD 15 can be formed entirely on the substrate 14, an advantage is that, it is unnecessary to align emissive layers with mask. Pixels of the organic electroluminescent display 10b are defined by a plurality of thin film transistors (not shown) and the color filters 16. Thereby, a rise in process yield or in resolution is made possible. However, it is difficult to produce the white OELD 15 that can emit “pure” white light.
Referring to FIG. 2A and FIG. 2B, the white OELD is usually binary or ternary structure. A white OELD 15a of FIG. 2A has two emissive layers such as a blue emissive layer 151 and a yellow emissive layer 152. A white OELD 15b of FIG. 2B has three emissive layers, such as a red emissive layer 153, a green emissive layer 154, and a blue emissive layer 155. When a hole combines with an electron to generate an exciton in different emissive layers, the different color lights are generated and mixed to produce white light. A disadvantage is, the distribution of the excitons is easy to change with the electrical field, so the color shift is made possible.
FIG. 1C shows that an organic electroluminescent display 10c fabricated by CCM replacing the color filters to absorb blue light. As shown, a blue OELD 17 with high light intensity is produced as a backlight having a blue emissive layer 171. The green and red lights are generated from the material 18 which can absorb blue light and then convert it efficiently. An advantage is that the process yield increase because the blue OELD is easier to produce than the white OELD. The reason is that the blue OELD has only one peak wavelength, but the white OELD has a plurality of peak wavelengths. However, the disadvantage is the emergent light of each pixel of the organic electroluminescent display 10c may scatter to mix with different color light of other pixel, so as to cause a color shift. A solution is to provide an external color filter (not shown) that may keep the color purity. In this way, the light need to be filtered twice, so the luminance and the efficiency reduce significantly. As usually, the CCM is hard to develop due to the requirement of high color purity and high efficiency, so the full-color OELD technology according to the CCM is rarely used in the industrial circle.
FIG. 3 is a spectrum of a blue OELD, each of whose layers is described as follows: anode (ITO)/hole injecting layer (CuPc)/hole transporting layer (NPB)/blue emissive layer/electron transporting layer (Alq)/electron injecting layer (LiF)/cathode (Al). The abscissa of the spectrum is light wavelength; the ordinate is light intensity. As shown, the blue OELD has about 456 nm of the major wavelength in emergent light. Without adding the CCM, the intensity of red and green lights are weak.
The present invention teaches a novel full-color display technology to overcome the shortages of forgoing three conventional organic electroluminescent display. The new technology employs less photomasks for simpler fabricating process, and implements electroluminescence to generate only one wavelength of the light. And also, the color shift and the color impurity are improved.